Technical Field
The invention relates to color printing. More particularly, the invention relates to color print management.
Description of the Background Art
State of the art color management systems try to solve all of the challenges of color management in the context of printing in a few operations and with minimum color definition. However, a color management workflow for printing must address the following operations:                Gamut adaptation between the source space and the printing space;        Color separation strategy, especially if the color space has more than three dimensions, such as CMYK;        Linearization for each ink to make response similar to a fixed standard;        Ink limitations, such as maximum ink volume per channel and in (n) dimensions of combinations per channel; and        Image and/or color adaptation, e.g. manual correction with software or fixed correction with algorithmic functions and/or look-up tables (LUT).        
While some of the foregoing operations could be included in one color LUT, such as an ICC profile, such technology is static. If it is desired to change or make a dynamic gamut adaptation, then all of these operations must be separated.
Further, for all color printing processes, it is necessary to decompose the color image information to be printed in each printing unit with respect to:                The colorimetric inks, also referred to as primary colors; and        Print settings, such as frame sequence, support, finishing, etc.        
Such decomposition of the color image is referred to as separation. The best known separation is CMYK (Cyan, Magenta, Yellow, Black), but separation can also involve other primary colors, e.g. Blue, Brown, Yellow, Black, and/or involve more than four primary colors, e.g. Hexachrome C, M, Y, K, Orange, Green. The final expectations of the artist/client provide a good correspondence between the image to be used for printing, as validated before printing, and the image that results after printing.
There are solutions that enable digital proofing, e.g. on paper, of separate files through a clear definition of colorimetric mixture layers. Generally used technology involves the use of a look-up table (LUT) for storing values in a profile connection space (PCS), such as the XYZ or CIELab color space, for example in an ICC color profile or equivalent. Each change in values of a layer in percentage (%) of ink has a repercussion on the final color. To adjust the aesthetic image in the context of its final result, it is common to change the channel values, e.g. CMYK, with editing software and visualization, for example by use of an editing program, such as Adobe Photoshop. In the CMYK space, operators usually have experience doing this and the correction is easy to understand because it is based on three primary colors and the impact of such primary colors on complementary colors. For example, Red consists of Magenta and Yellow. Changing the Red, in turn, acts on the information concerning Magenta and Yellow. For further example, clarity is often defined by the layer of Black and/or by the combined action of three layers trichrome (C, M, Y).
Changing the separation values, e.g. CMYK or nCLR (≥4 CLR) for aesthetic reasons can lead to problems during printing. For example, the operator can increase the total ink (TIL: Total Ink Limit) and create problems with drying and/or with the inks that are required. Thus, in the case of use of color away from traditional CMYK primaries, for example Blue, Red, Green, Yellow, the actions necessary to retouch the image are different from those known by experience and the learning process is long and must be repeated for each new configuration of ink.
In the case of a separation of more than four colors (nCLR), for example using as the four first colors, colors that are similar to those of the CMYK color space, even if the colors are different, the correction on separate layers becomes very complex for the operator. For example, the color “flesh” in Hexachrome OG may involve Orange, Yellow, Magenta, Black, and Cyan. Inappropriate modifications of the Orange or Magenta or Yellow layer can cause visible artifacts in the image.
Further, the color effect for data types that are achromatic such as, for example white ink for printing on a colored support, e.g. type Brown cardboard; transparent varnish matt, gloss, satin, etc.; and metallic ink, e.g. Silver ink, are not very easily visible if the separation algorithm achieves results that are perceived as natural and qualitative by the observer. For some creative operations, it may be necessary for the creative work on, for example a virtual file as disclosed herein to view the presence, location, and quantity of a particular ink, i.e. Silver ink, before separation, given that the amount of this ink is automatically calculated by subsequent separation technology, based on color information defined in the virtual space, combined with a strategy of color separation (CSS: Color Separation Strategy). Silver ink, for example, when viewed at certain angles between the light and the observer, is seen as having a color medium gray, as a gray ink of the same color, but without the specular effect made by the metallic pigments. In virtual space—with only one image or with a static image—it is difficult, when visualizing a 2D image on a screen, to determine if the color medium gray is a shade of black ink or if it is a shade of Silver ink.
Additionally, when the chromatic adaptation and management of out-of-gamut colors, for example all the values that must be separated, are in gamut there is a unique relationship between a color of type CIELab, XYZ, or equivalent and a space (n) dimension, i.e. if colors are in-gamut then there is a unique relationship between the device independent color space (CIELab, XYZ) and colorants. Note that this is not true if there are more than three colorants. Different CMYK combinations can have the same CIELab or XYZ. However, for purposes of the invention herein, this is true in the disclosed virtual space. The use of an encoding of type LCh (Lightness/clarity-Chroma/Saturation-hue/tint) is in compliance with a representation of color space and can result from a number of decisions based upon experience or from analysis of colorimetric data. One problem comes from the fact that it is difficult to establish relationships of colors, depending on the pigment inks, the color of the substrate, etc., when the color source, working from data coded L*C*h, is an area of size that is variable and not constant.
Finally, for any printing process which has a post-printing operation (PPO), such as varnish, lamination, kiln, etc., the printer operator and the customer cannot decide in advance of the PPO if the printing result achieved after the PPO will be correct. In such case, all of the adjustments that concern color, e.g. density, dot gain, etc. are made during the printing process and give a visible result before the PPO. Usually, the PPO is not available immediately but, rather, is only available some hours or days afterward. However, the PPO typically generates some color differences, e.g. kiln influences for ceramic substrates, or influences the human perception of the color, for example due a glossy difference. The customer wants to have a final production, i.e. after the PPO, in accordance with his artwork and/or physical proof, based upon a visual comparison between the current print and the reference proof. Unfortunately, the printer proof only shows the final result, and not the result before the PPO.